Pressurized gaseous fuels are supplied to a combustion process in a number of applications such as, for example, high velocity oxyfuel thermal spraying. A typical pressure required for such operations is about 100 psig and flow rates of up to about 200 standard cubic feet per hour (SCFH) per spray gun. The fuels used in such systems include propylene, propane, and methyl acetylene propadiene (MAPP), in combination with oxygen.
The liquid fuels are stored in pressurized containers. When the container is at least substantially filled, the liquid fuel is ejected as a gas under vapor pressures which are sufficient to employ the same in high pressure, high flow rate spraying applications. The temperature of the liquid fuel decreases due to the latent heat of vaporization thereby reducing the pressure within the container. As a result, it is difficult to maintain a constant pressure and constant flow rate of the gas. It is not uncommon under these circumstances for the operation of the storage container to be discontinued until the temperature within the container rises to a level sufficient to provide the necessary vapor pressure for the gas. The most desirable vaporized streams are those that are supplied to the combustion system at a constant pressure, constant flow rate and a constant gas composition.
Maintaining a constant pressure and flow rate presents problems. This is because as the level of the liquid fuel in the storage container decreases, the temperature likewise decreases due to the latent heat of vaporization. A reduction in temperature within the storage container results in a reduction of pressure which adversely affects both the pressure and flow rate of the vaporized fuel stream.
It is not uncommon under these circumstances to discontinue the operation of the storage container until the temperature within the container rises to a level sufficient to provide the necessary vapor pressure for the gas. Such an arrangement, however, is inefficient because the vapor fuel supply is discontinuous. In order to overcome this problem, storage containers are combined in groups with some of the containers (typically one-half) supplying fuel while the other containers are off-line until reaching an operating temperature. Although this system provides for the continuous supply of the fuel gas, it is costly and inefficient.
It has been proposed to maintain the pressure within the storage container by supplying the container with a stream of the vaporized fuel from an external source, such as from the vaporizer used to heat the liquid fuel to form a vaporized stream. However, when the heated vaporized stream is transported to the generally cooler storage container there results a change in the composition of the vaporized stream, if the fuel comprises more than one component (i.e. a mixed fuel). This is due to the different vaporization temperatures of the components of the mixed gas. For example, a liquid fuel containing 94% propane and 6% butane will generate a vaporized stream that will vary in composition with respect to both propane and butane components.
The change in composition of the fuel gas presents problems in the combustion process. Changes in the composition of the fuel gas can result in the presence of uncombusted hydrocarbons leading to the formation of soot within the ejection nozzle. In particular, a change in the vapor composition to one containing a higher fraction of higher hydrocarbons, changes the amount of oxygen needed for complete combustion. This may require more oxygen which adds to the cost or varying rates of oxygen which is difficult to maintain.
The use of liquid fuels stored in customary storage containers therefore suffers from disadvantages which adversely affect both the flow rates and composition of the fuel. As a consequence processes requiring high pressure, high flow rate vaporized fuel streams such as oxyfuel thermal spraying are inefficient and costly.
It would therefore be a significant advance in the art of generating high pressure, high flow rate vaporized fuel streams if a fuel stream could be produced at a substantially constant pressure and flow rate without substantial variation in composition.